Shear viscosity of neutron-rich nucleonic matter near its liquid-gas phase transition

TL;DR

This study investigates the shear viscosity to entropy density ratio of neutron-rich nucleonic matter near its liquid-gas phase transition, revealing a valley-shaped dependence and a sharp drop at the first-order transition.

Contribution

It introduces a relaxation time approach with in-medium nucleon masses to analyze shear viscosity near phase transitions in neutron-rich matter, highlighting phase transition effects on viscosity.

Findings

Shear viscosity ratio shows a valley shape near the phase transition boundary.

A sharp drop in $\, ext{eta}/s$ occurs at the first-order phase transition temperature.

The ratio varies smoothly across the second-order phase transition.

Abstract

Within a relaxation time approach using free nucleon-nucleon cross sections modified by the in-medium nucleon masses that are determined from an isospin- and momentum-dependent effective nucleon-nucleon interaction, we investigate the specific shear viscosity ($\eta/s$) of neutron-rich nucleonic matter near its liquid-gas phase transition. It is found that as the nucleonic matter is heated at fixed pressure or compressed at fixed temperature, its specific shear viscosity shows a valley shape in the temperature or density dependence, with the minimum located at the boundary of the phase transition. Moreover, the value of $\eta/s$ drops suddenly at the first-order liquid-gas phase transition temperature, reaching as low as $4\sim5$ times the KSS bound of $\hbar/4\pi$. However, it varies smoothly for the second-order liquid-gas phase transition. Effects of the isospin degree of freedom and the nuclear symmetry energy on the value of $\eta/s$ are also discussed.